<!DOCTYPE  html>
<html>
<head lang="en">

</head>
<body id="change">

    <a href="ex1/index.html">
        <button>ex1</button>

    </a>
    <a href="ex2/index.html">
        <button>ex2</button>

    </a>
    <a href="ex3/index.html">
        <button>ex3</button>

    </a>
    <a href="ex4/index.html">
        <button>ex4</button>

    </a>

</body>
</html>